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REVIEW OF 
LOCALISATION MOTRICE ET KINESTHESIQUE 



HENRY H. DONALDSON 



Reprinted from The Journal of Nervous and Mental Disease, Vol. 39 

No. 1, January, 191 2. 






[Reprinted from The Journal of Nervous and Mental Disease, Vol. 39, 

No, 1, January, 191 2.] 



Localisation Motrice et Kinesthesique (Les noyaux masticateur et 
mesencephalique du trijumeau chez le lapin). Par Edouard Willems, 
Assistant a l'lnstitut d'Anatomie de l'Universite libre de Bruxelles 
(Institut Warocque). Extrait de la Revue " Le Nevraxe," Vol. 
XII, 191 1, pp. 9-224. 

In this study Willems is concerned with the structure and functions 
of the nucleus radicis descendentis (noyau mesencephalique) and the nu- 
cleus motorius (noyau masticateur) of the nervus trigeminus, as these 
appear in the rabbit. By way of introduction he gives an excellent out- 
line of the literature touching these nuclei. His findings are briefly as 
follows : 

The nucleus radicis descendentis of one side contains 1,578 vesicular 
cells (p. 108) arranged in a column lying at the junction of the dorsal and 
ventral plates (fig. 4, p. 40). This column extends caudad from the level 
of the thalamus in the fetus — or the caudal edge of the colliculi superiores 
in the adult — to the level of the nucleus motorius. In their general ap- 
pearance these cell bodies are similar to those in the spinal ganglia. 

The nucleus motorius is composed of an oval mass of 2,942 cells — 
2,642 large and 300 small (p. 108) — which have the typical characters of 
efferent neurones of the first order. This nucleus is situated in meten- 
cephalon at the level of the emergence of the portio minor of the nervus 
trigeminus. The total number of cell bodies in both nuclei (of one side) 
is therefore 4,520. 

The portio minor of the nervus trigeminus contains 4,859 medullated 
fibers (the average of four determinations on fully grown animals). 
This total may be further analyzed into 3,014 fibers of large diameter and 
1,845 of small diameter (Table IV, page 147). 

From the relation between the number of cells in the two nuclei com- 
bined (=4,520) and the number of medullated fibers (=4,859) in the 
portio minor, Willems concludes that all the fibers from both nuclei unite 
in this division of the nervus trigeminus, there being no other source from 
which these fibers could come. He has observed further the splitting 
(i. e., division into two or more) of the fibers arising from the cells of 
the nucleus, radicis descendentis before these enter the portio minor and 
this phenomenon, so far as it goes, would help to explain the excess in 
the number of fibers observed. 

It is further possible that the number of small cells credited to the 
nucleus motorius has been underestimated — which would also help to 
reduce the disparity in the two enumerations — although as the numbers 
show, this disparity amounts to an excess of only 339 fibers, or about 7 
per cent, of the total number of cells. 

The portio minor of the nervus trigeminus supplies in the rabbit nine 
trigeminal muscles (p. 24). 

The masseter, pterygoideus internus, sphenoidalis (distinguished and 
described for the rabbit by the author) pterygoideus externus. digastricus, 

67 



68 BOOK REVIEWS 

(anterior belly), temporalis, mylo-hyoideus, tensor palati and tensor 
tympani. 

All the muscles in the domain of the nervus trigeminus are in the 
last analysis derived from the adductor or elevator of the mandibular arch 
of fishes (p. 31). 

Excepting the digastric muscle — which receives about 200 fibers from 
the trigemino-facial plexus (p. 172) — all the muscles named above receive 
their entire innervation from the portio minor. 

The evidence for this statement is based on the distribution of the 
portio minor as shown by dissection and on the chromolysis in the two 
nuclei following excision of the muscles or extraction of the nerves sup- 
plying them. 

In the nucleus motorius a study of those cell groups which undergo 
chromolytic changes after operation makes possible a very complete recon- 
struction of the entire nucleus. Its different portions can thus be assigned 
to given muscles. 

On the other hand, for causes not yet known, the chromolytic reac- 
tion can be obtained simultaneously in only about one half the cells which 
constitute the nucleus radicis descendentis (i. e., in 815 out of 1,578 cells). 
Moreover, the cells which do react are not characteristically grouped for 
the several muscles. 

Why the " axone reaction " does not appear in all of the cells of this 
nucleus is not at the moment clear. Nevertheless it is important to note 
that it does appear very evidently in more than half of the cells consti- 
tuting the nucleus. It is therefore proper to emphasize the fact that there 
is conclusive evidence that fibers from both nuclei are distributed to sev- 
eral of the muscles, notably the masseter, sphenoidal, temporal and tensor 
tympani, while on the other hand there is some evidence— though less 
satisfactory — that the innervation of the remaining five muscles is also 
from both nuclei. It follows that certainly the four muscles above named 
— and probably all nine — receive a double nerve supply. 

Willems is of the opinion that the fibers in the portio minor of one 
side are all from homolateral cells. 

The central connections of the two nuclei are not of the same type. 
The cells of the nucleus motorius have characteristic motor connections, 
while those forming the nucleus radicis descendentis do not have motor 
connections, but on the contrary are largely associated with secondary 
sensory pathways. The distinction is sharp. 

The histological differences between the two nuclei are also notable. 
In the nucleus radicis descendentis the vesicular cells — consisting of a 
large and a small form— give rise to but few minor dendrites. One main 
dendrite, however, leaves the cell to pass in the direction of the bundle 
of fibers coming from the nucleus motorius. 

These outgrowths sometimes split and generally exhibit a rapid de- 
crease in diameter as they pass away from the cell body — the so-called 
" conical diminution." 

Moreover, these outgrowths arise without any initial constriction such 
as is characteristic of the axone, but, on the other hand, always give rise 
to one or more small branches which do have the initial constriction and 
general appearance of axones. Many of these latter pass to the cells of 
the nucleus motorius and there terminate about them. 

In contrast to this arrangement, the cells of the nucleus motorius 
have typical dendrites and axis cylinder processes, which latter form a 
well-defined root bundle that passes out in the portio minor, 



GHfl! 



BOOK REVIEWS 69 

Aside from the axones, coming from the nucleus radicis descendentis, 
there end around the cell bodies and dendrites of the neurones forming 
the nucleus motorius a mass of terminals derived from a number of 
other sources. 

Many of the foregoing observations evidently suggest that the nucleus 
radicis descendentis is sensory in function. 

If that were the case, then we might expect that its fibers in the 
portio minor and to the several muscles would be present in about the 
same proportion as the sensory fibers in the muscular nerves of other 
mammals. 

If the number of fibers from each nucleus corresponds with the 
number of cells in each, then about 35 per cent, of the fibers would come 
from the nucleus radicis descendentis. This is just above the lower limit 
for the number of sensory fibers in the muscular nerves (cat) as given 
by Sherrington (.'94-95 ) (p- 138). 

On the other hand, the axone reaction shows that the masseter, 
sphenoidal and temporal muscles are connected with 1,199 cells in the 
nucleus motorius and 800 cells in the nucleus radicis descendentis. Thus 
the latter are about 40. per cent, of the total number. Therefore about 40 
per cent, of the fibers in these cases might be regarded as coming from 
the nucleus radicis descendentis. 

It appears then that there is nothing in the numerical relations of 
the fibers from the two nuclei which opposes the idea that those from the 
nucleus radicis descendentis are sensory in function. 

From his findings, of which the foregoing is but a bare outline, 
Willems makes the following argument : 

1. With the partial exception of the digastric, above noted, the 
muscles in the domain of the nervus trigeminus receive all their fibers 
from the nucleus radicis descendentis and the nucleus motorius. 

2. There is no instance known w r here the motor fibers going to a 
muscle arise from two separate nuclei diverse in structure and connections 

3. These two nuclei are plainly diverse in structure and connections. 

4. The nucleus motorius is admittedly motor in function. The func- 
tion of the nucleus radicis descendentis has heretofore been in doubt. 
Willems concludes that it is sensory, mediating muscular sensibility. 

In more detail the reasons for this conclusion are the following : 

The cells of the nucleus radicis descendentis are histologically similar 
to spinal ganglion cells. The axones, arising from the main outgrowth 
(dendrite), which forms the peripheral fiber, pass in large measure to the 
cells of the nucleus motorius and end about them. 

The main outgrowths enter the portio minor and are distributed with 
the fibers from the nucleus motorius to the several muscles. Their pro- 
portional representation is that of the sensory fibers in a muscular nerve. 
The central connections of the cells of the nucleus motorius are with 
motor tracts. The central connections of the cells of the nucleus radicis 
descendentis are mainly with the nucleus motorius, but also with some 
secondary sensory pathways. 

As the sensibility of muscles is mediated by afferent nerves, it seems 
most probable that the nucleus radicis descendentis is a sensory nucleus 
mediating muscular sensibility. These neurones are then afferent neurones 
of the first order, homologous with the neurones forming the ganglia of 
the cerebral or spinal nerves. The group is however unique in mammals 
in that it forms a nucleus having a single sensory function and also in 



70 BOOK REVIEWS 

that it is permanently included in the wall of the neural tube. These 
cor. : are well founded and the paper constitutes a contribution of 

Gist-class importance to our knowledge of the mammalian nervous system. 

For the establishment of these conclvsiir.s. quantitative tests have 
been largely used, and in this connection the author expresses apprecia- 
tion of the quantitative work on the nervous system which has been pub- 
lished during the last fifteen years by American authors in the Journal 
of Comparative 1'eitrology and Psychology. Since all of the author's 
applications of the quantitative tests did not bear directly on the main 
argument, some were not mentioned in the foregoing outline, but before 
dosing this review we wish to comment on several of these, since they 
are important for our notions of the general architecture of the nervous 
system. 

We shall discuss three points c 

(a) The splitting of fibers in their peripheral course. 

(b) The classification of fibers in the portio minor, according to 
diameter. 

, (c) The relation between the weight of a muscle and (i) the number 
or (2) the diameter of the fibers passing to it. 

(a) The fact that fibers split or divide in their course has been long 
known but the large proportion of splitting fibers has been appreciated 
only recently, and even yet the fact has not received due consideration in 
the text-books. For example. Dunn ( '99, '02) showed that in the nerves 
to the frog's leg, splitting occurred in 10 per cent, of the fibers going to 
the thigh and 22 per cent, of those going to the shank. Dunn (09) 
further showed in the case of a frog in which the legs were supplied by 
the sensory fibers alone, that practically the same amount of splitting 
occurred: namely, 10 per cent, in the thigh and 28 per cent, in the shank. 
Thus among the fibers going to the leg :: the frog, both the sensory and 
motor split in considerable numbers. 

From the physiological standpoint, the idea of splitting motor fibers 
meets no great obstacle, but the splitting of sensory fibers runs counter 
to the established doctrine of " local signs " in its usual form, and is there- 
fore less readily accepted, although it may be noted that it seems to offer 
an anatomical basis for at least some cases of _** referred pain." 

In the study of the portio minor, Willems, dealing with both sensory 
and motor fibers together, finds that the combined branches of the portio 
minor conta::". 5,564 fibers as contrasted with 4859 fibers in the trunk at 
the point of emergence. Thus there is an increase of 703 fibers or 14.4 
per cent. (Table IV, p. 147) due to splitting. 

At the same time he observed within the metencephalon, as previously 
stated, a splitting of the fibers arising from the cells of the nucleus radicis 
descendentis, thus contributing a new observation on the splitting of sen- 
sory fibers. 

(b) In examining the cross-section of the portio minor, Willems finds 
fibers of both large and small diameter, and raises the questions of their 
grouping and significance. 

Touching the grouping, it is desirable to form an opinion as to whether 
these fibers fall into two groups, the large and the small, or form a graded 
series from large to small (p. 139). 

Boughton (06) maintained in the case of the purely motor oculomotor 
nerve (of the rat and cat) the division into two groups, and the data of 
Sherrington C94-95) are susceptible of a like interpretation, although 
not so interpreted by Sherrington himself. 



BOOK REVIEWS 7 1 

Willems found in the portio minor of the adult (see Table IV, p. 
147, mean of. last four records from mature animals) an average of 3,013 
large and 1,845 small fibers, the small fibers being therefore 38 per cent, 
of the total. 

On the other hand, he found in the case of the rabbit ten days old, 
an average of 2,493 large fibers and 773 small — the small fibers thus rep- 
resenting 23 per cent, of the total number. Since in the adult the cells of 
the nucleus radicis descendentis are about 35 per cent, of the sum of the 
cells in the two nuclei combined, and since the sensory constituent of a 
mixed nerve is usually credited with the great majority of the small fibers, 
Willems concludes that the small fibers form a recognizable group — that 
they come mainly from the nucleus radicis descendentis, and that their 
small diameter is additional proof of their sensory function. The relative 
number of small fibers appears to increase after birth, because, according 
to Willems, medullation takes place in them as a class at a later date than 
in the motor fibers. This argument is not convincing. 

Boughton's observations were made on a purely motor nerve — the 
oculomotor — the sections being taken only shortly distad of the point of 
emergence. In this nerve there is a tendency for the fibers to appear in 
two groups — distinguished by their average diameters. 

During post-natal growth the relative number of the small fibers 
increases, but these small fibers never become large fibers. Thus all of 
the characters which Willems uses to distinguish the sensory from the 
motor fibers in the portio minor occur in a nerve containing motor fibers 
only. In this connection Willems criticises, as without foundation, Bough- 
ton's statement that the small fibers (in the oculomotor) are those which 
" come in after the period of most rapid growth." The criticism is too 
severe. Probably many of the small fibers are present as unmedullated 
axones from an early period, but that some of the axones do grow in 
later is rendered probable from such observations as those of Ranson 
('04) on the fibers which grow across the site of a lesion in the corpus 
callosum of the albino rat. 

To sum up this matter it does not appear probable that Willems' gen- 
eral interpretation of diameter in relation to function in the case of the 
portio minor is correct, and it is also evident that the observations of 
Boughton cannot be used to support his conclusion. 

(c) (1) Willems finds in general that the number of fibers per unit of 
muscle weight tends to increase as the muscle becomes lighter (= smaller). 
For the first seven muscles this increase is moderate, i. e., from 3.8 fibers 
per centigram of muscle in the heaviest muscle — the masseter — and 2.8 
for the next heaviest — the pterygoideus internus — it increases regularly to 
9 fibers in the mylohyoid, while in the case of the two remaining muscles, 
the tensor palati has 23 and the tensor tympani 333 fibers per unit of 
weight. This determination is unique, so that there are no observations 
with which it can be fairly compared. 

Donaldson ('03) showed that the number of motor fibers passing to 
the muscles of the thigh and of the shank of the frog's leg are distributed 
to these divisions of the leg in proportion to the weight of the muscles. 
But as to the number of fibers to the individual muscles of known weight. 
we have as yet no data and hence the relations in this case do not bear on 
those found by Willems. 

(c) (2) Touching the last point, on the relation between the diam- 
eter of the medullated fibers and the muscles which they supply, the 
existing observations are as follows : 



72 BOOK REVIEWS 

Schwalbe ('81) from a study of the diameters of the nerve fibers to 
the arm and leg of the frog, concluded that the fibers of largest diameter 
had the longest course. This says nothing, however, concerning the diam- 
eter of fibers and the weight of the muscles which they supply, except 
by implication. 

Dunn ('99, '02), working on the frog, determined that among the 
fibers entering the frog's leg, those of largest diameter ended in the 
thigh, and among the remainder, those of largest diameter in turn ended 
in the shank. This showed that Schwalbe's inference was incorrect that 
as a matter of fact the larger fibers ran the shorter course, but again did 
not establish, except by implication, the relation of the diameter of the 
fibers to the individual muscles. 

Herrick ('02), however, did associate the diameter of the nerve fibers 
with the degree of development of the end organs, concluding that the 
more'functionally active end organs received the fibers of greater diameter. 

Willems finds that the distribution of the fibers of different diameter 
to the muscles innervated by the portio minor does not fit with any of 
the preceding views, expressed or implied. With the observation of Don- 
aldson ('03) which apply to segments of the leg, no direct comparison is 
really possible, nor can Herrick's criterion be applied, so that the failure 
of Willems's observations to fit with those just cited has no bearing on 
the correctness of the latter, but indicates merely the need of further work 
in order to make such comparisons possible. 

Willems's data do not show anything regular in the distribution of 
fibers of large diameter, but they do show, with the exception of the tensor 
tympani, that in the remaining muscles the proportion of small fibers tends 
to increase as the weight of the muscles increases, though more slowly. 

Willems's results in this field lead him to make some interesting sug- 
gestions on the possible relations between the diameter of nerve fibers and 
the secondary growth of muscles as indicated by the size of the muscle 
fibers — suggestions well worth further examination. 

Two general matters remain to be mentioned. The paper before us 
unfortunately contains a considerable number of misprints of all kinds, 
but especially misprints of numbers. Fortunately there is no instance, 
however, where these misprints seriously modify the argument, yet one 
consequence is that some of the numbers given in this review are different 
from those printed in the original paper. A carefully prepared list of 
corrections, in addition to the " errata " sheet which accompanies the 
paper, would add greatly to the usefulness of these observations. Graphique 
VI seems to have been omitted (see p. 128). 

It is interesting to note that the author feels his labors to have been 
increased and his results complicated by the fact that he was obliged to 
use rabbits of various and unknown breeds and of undetermined ages. 
Failure to get this last datum made it necessary for him ultimately to 
exclude three out of seven of his series of quantitative and numerical 
determinations as they had plainly been made on immature individuals. 

It is hardly necessary to enlarge on this topic, but since work of the 
sort here reviewed is bound to be more frequent in the future, it is evi- 
dent that, as a first step, standard strains of animals of known ages 
should be generally available for the purpose of such studies. 

Henry H. Donaldson. 



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